The helicase domain of Dicer ensures strict processing of evolutionarily optimized microRNAs

Ribonuclease Dicer generates small RNAs for RNA interference (RNAi) and microRNA (miRNA) pathways. Mammalian Dicer produces miRNAs but its tripartite N-terminal helicase domain inhibits processing of double-stranded RNA for RNAi. Mouse oocytes express Dicer, which enhances RNAi because it lacks the helicase’s proximal subdomain HEL1. Here we show that genetic removal of HEL1 in mice causes embryonic growth retardation, defects in the cardiopulmonary system, and perinatal lethality. HEL1 suppresses biogenesis of mirtrons, a non-canonical low-abundant class of miRNAs, and is required for high-fidelity cleavage and strand selection during biogenesis of canonical miRNAs. Essential is the HEL1 structure, not its helicase activity, because mutations of critical aminoacids do not affect viability or fertility and have minimal impact on miRNA biogenesis. Altogether, HEL1 is a critical structural component of Dicer in its role of a highly conserved structural “mold” that ensures high-fidelity processing and adaptive evolution of mammalian miRNA precursors. Bulk small RNA-seq of 11 mouse E15.5 embryo samples: - 3x WT - 5x DicerX homozygotes - 3x DicerX heterozygotes